Methods and systems for controlling the treatment of liquid comprising solid matter are known in the art.
Usually, a peak process load, i.e. peak hydraulic load and waste load, for a given treatment plant/basin will be determinative for the size of the mixer machine, which during operation is driven at full operational speed, i.e. directly connected to the power mains. Thereto, when the treatment plant is constructed/designed, the components such as the mixer machine are dimensioned for the peak process load that is expected for instance 10 or 15 years into the future. Thus, this way of designing a treatment plant and controlling the mixer machine in a treatment plant during anoxic and/or anaerobic conditions consumes an unreasonably large amount of energy.
A basin having anoxic and/or anaerobic conditions, may for instance be a Sequential Batch Reactor (SBR) during an anoxic and/or anaerobic process step, or be a separate basin designed for an anoxic and/or anaerobic process step in a line of basins which are designed for different process steps.
Some known systems having a mixer machine equipped with a synchronous motor also comprise a variable speed drive in order to be able to start the mixer machine by ramping up the speed from zero to full operational speed, since a synchronous motor due to its design cannot be directly started as full operational speed. The operational speed is then kept at full operational speed.
A development in the field of controlling treatment of liquid comprising solid matter is described in U.S. Pat. No. 8,323,498, that discloses a system for automatic control of a treatment process, i.e. aeration and mixing devices, based upon at least one dynamically measured parameter. According to U.S. Pat. No. 8,323,498 the system determines if the treatment process is aerobic or anoxic, respectively. When the treatment process is determined to be anoxic the aerators are turned off and the mixer machines are driven at the operational speed required by the real-time process load. Thus, U.S. Pat. No. 8,323,498 is entirely based on the idea that the mixer machines must be active, i.e. kept in one and the same operation mode, all the time during the entire anoxic process step in order to have a useful treatment. The mixer machine is driven at this process load dependent operational speed, also called the process speed, all the time during the anoxic process step until the treatment process once again is changed to aerobic. Thus, the process speed of U.S. Pat. No. 8,323,498 is an influent process load depending process speed, and especially dependent on the waste load/concentration in the influent wastewater, instead of using time dependent operation, in order to save wear and energy. During the entire anoxic treatment step, the process speed of the mixer machine must never go below the speed needed to keep the solid matters suspended.
Documents US 2010/243558 and US 2014/138308 are directed towards an alternative (non-combinable) treatment philosophy than the above mentioned U.S. Pat. No. 8,323,498. Instead of having the mixer machine active during the entire anoxic process step (as stipulated by U.S. Pat. No. 8,323,989), documents US 2010/243558 and US 2014/13808 disclose an alternative treatment philosophy using intermittent operation of the mixer machine between an active and an inactive operation mode, respectively, during the anoxic/anaerobic treatment step in order to save energy.